Leo Valdez saved the world. As reward, he made a simple request.
To free Calypso, the love of his life, from her island prison.
Now, four years later, Calypso, nicknamed Callie by everybody outside the Seven, lives happily with Leo in their home of Texas, working in their machine shop.
Calypso hasn't had contact with the modern world, besides the occasional lover, in millennia. Even after four years, she still hasn't quite mastered how to talk to other girls. Men, sure, but not girls.
Leo and Calypso have a long way to go for their happily ever after, even if they don't know it yet. Aphrodite promises it.
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Will probably change, since we don't have a definite plot.
When the gods do not keep their promise and Calypso dies in Leo's arms for leaving the island, Leo comes back to CHB a changed person. He doesn't crack jokes anymore, he ignores his friends and isolates himself from everyone.
But most importantly, he wants to take revenge on the gods, and no one was going to get in his way.